1. Field of the Invention
This invention relates to magnetic bubble domain devices, and more particularly to a bubble storage device and a transfer switch for transferring bubble domains betwen different shift registers, where the transfer switch can be made by a single level masking process and is particularly suitable for reliable transfer of very small bubble domains using very low currents.
2. Description of the Prior Art
Magnetic bubble domain devices are well known in the art, and in many of these devices it is necessary to tranfer bubble domains from one shift register to another. For example, in a major/minor loop type of memory organization, such as is shown in U.S. Pat. No. 3,618,054, bubble domains are transferred between the input/output major loop and the storage minor loops.
Many devices have been described in the prior art for transferring bubble domains from one shift register to another. However, as the size of the magnetic bubble decreases, the design of a proper switch becomes more difficult. Thus, while many current controlled transfer switches are described in the prior art, it is difficult to find one which will work properly when bubble domains of very small size, such as one micron and less in diameter, are used.
In general, a good current controlled transfer switch is one which has very low current amplitude requirements even when extremely small bubble domains are used. This is very important since, as the size of the bubble domains decreases, the linewidth of elements used to move the bubbles also decreases and, if the currents required are too large, problems such as electromigration will occur.
Additionally, a good transfer switch should be compatible with conventional propagation elements used to move bubble domains in shift registers, and should be such that its design is compatible with the design used for the propagation elements in the shift registers. Further, a switch which can be used for all functions on the magnetic chip is desirable. A switch capable of symmetrical transfer should operate with the same margins regardless of the direction of transfer. Another important criterion for a good transfer switch is that it provide good margins for switching and be reliable in its operation regardless of the manner in which it is fabricated. Desirably, it should be capable of being fabricated by single level metallurgy processes using only one critical masking step. Still further, it is desirable that the transfer switch be an integral part of the propagation structure used to move magnetic bubble domains.
In the prior art, various bubble domain transfer switches are described using current carrying overlays to switch bubble domains from one propagation track to another in response to an electrical signal pulse. Typically, the current carrying conductors are designed in a loop configuration so that a current pulse in the conductor will generate a localized magnetic field within the loop. This localized field temporarily adds to the fields of the propagation elements in that region and provides an additional attractive or repulsive force on magnetic bubble domains approaching that region. In this manner, the bubble domain is preferentially attracted or repulsed in order to determine the propagation track along which it will move. A representative example of such a switch is shown in the IBM Technical Disclosure Bulletin, Volume 15, No. 2, July 1972 at page 703. In that switch, bubble domains arrive at a point of ambiguity where two possible positions are available for subsequent bubble domain movement. Current in a conductor determines which path will be taken by the bubble, thus resolving the ambiguity of the switch.
A replicate type of switch using a current carrying loop is described by Bobeck et al in IEEE Transactions on Magnetics, Vol. MAG-9, No. 3, September 1973, at pages 474-480. In this type of switch, current is used to sretch a bubble domain so that it will transfer to a different propagation channel, while an additional amount of current is used to replicate the stretched domain. This type of switch requires high currents and is difficult to incorporate in a single level metallurgy design.
Another version of the replicate switch described in the previous paragraph has been shown by T. J. Nelson, in AIP Conference Proceedings, 18, 95(1974). This is an all-permalloy switch in which a current conducting path is comprised of permalloy deposited at the same time as the permalloy propagation elements. Transfer from one propagation channel to another utilizes a current along a straight permalloy path linking the propagation channels. A disadvantage of this transfer switch is that very high currents are required when the bubble diameter is small, of the order of one micron and less in diameter. Physically, this switch is large and thus is not compatible with densely packed major/minor loop designs.
Still another transfer switch employing a current carrying conductor is shown in U.S. Pat. No. 3,876,995. A double loop conductor is used to establish a magnetic field which attracts a bubble toward one propagation track and at the same time establishes another magnetic field tending to repel the bubble from another propagation track. Additionally, this patent shows a transfer switch which uses no current. Instead, two bubble propagation tracks merge at a junction.
In order to overcome the disadvantages of the prior art transfer switches, a new design for a transfer switch is described herein. The switch can be fabricated using single level metallurgy, and is particularly advantageous when very small magnetic bubble domains are to be transferred. The switch uses very little current even for transfer of very small magnetic domains, and the structure of the magnetic elements comprising the switch is compatible with propagation elements presently used, so that the transfer switch itself can be integral with a propagation path.
Accordingly, it is a primary object of the present invention to provide a transfer switch for magnetic bubble domains, which can be made by single level metallurgy and which requires very small currents for transfer, even when the magnetic bubble domains are very small.
It is another object of the present invention to provide a transfer switch for magnetic bubble domains which is an integral part of the propagation structure used to move such domains, and which provides reliable transfer of the bubble domains.
It is another object of the present invention to provide a switch for transferring magnetic bubble domains between one shift register and another, where the switch provides transfer with very good margins in all propagation paths through the switch.
It is another object of the present invention to provide a magnetic bubble domain transfer switch which can be fabricated by single level metallurgy, and which does not require a transfer current carrying conductor having a loop therein, where the transfer switch can be used with many types of known propagation elements.